1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine, and more particularly to a system which performs a duty ratio control of an open/close valve disposed in an air intake side secondary air supply passage.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are well known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and the air-fuel ratio of the mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purpose of purification of the exhaust gas and improvements of the fuel economy. As an example of the air-fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533, in which an open/close valve is disposed in an air intake side secondary air supply passage leading to the carburetor on the downstream side of the throttle valve, and the open/close valve is on-off controlled in response to the output signal level of the O.sub.2 sensor, so as to effect a "duty ratio control" of the supply of the air intake side secondary air. In the system mentioned above, whether the air-fuel ratio of the mixture supplied to the engine is richer or leaner than a target air-fuel ratio is detected in response to the output signal level of the O.sub.2 sensor. When the result of the detection is that the air-fuel ratio of the mixture is leaner than the target air-fuel ratio, an opening period of the open/close valve within the period of one duty cycle is decreased by a predetermined decremental amount. On the other hand, when the result of detection is that the air-fuel ratio of the mixture is richer than the target air-fuel ratio, the opening period of the open/close valve within the period of one duty cycle is increased by a predetermined incremental amount.
However, the O.sub.2 sensor used in this type of system generally has a characteristic such that a speed of response under a condition where the air-fuel ratio of the mixture varies from a rich side toward a lean side is slower than a speed of response under a condition where the air-fuel ratio varies from the lean side toward the rich side. Therefore, it has been difficult to control the air-fuel ratio by simply adjusting the valve open time period by a same correction amount without regard to the detected state of the air-fuel ratio of the mixture, i.e., whether the detected air-fuel ratio is lean or rich.
Further, in the air intake side secondary air supply systems, the operation of the system is such that the air-fuel ratio of the mixture to be supplied to the engine is controlled toward a predetermined target air-fuel ratio. Therefore, it is desirable to set the target air-fuel ratio at a value greater (leaner) than a stoichiometric value within a level which does not cause a misfire, and to effect a PI type control of the supply of the air intake side secondary air, which is a combination of a proportional type control and an integral type control.
However, if a correction value of an integration term or a proportional term is determined to be the same in both cases where the air-fuel ratio is controlled toward the lean side and the air-fuel ratio is controlled toward the rich side, it is very likely that the control operation from the lean side to the rich side is delayed due to a time period from a point at which the air-fuel ratio of the mixture is controlled in the air intake system by the supply of the secondary air to a point of time at which a result of control operation is detected as a change in the oxygen concentration in the exhaust gas. Therefore, a problem of the delay of response has to be solved for providing good driveability of the vehicle.